Recently, as a battery that meets the expectations for miniaturization, lightness, and high capacity, a non-aqueous electrolytic solution-based secondary battery such as a lithium ion battery has been suggested and put into practical use. The lithium ion battery includes a positive electrode and a negative electrode which have properties capable of reversibly intercalating and deintercalating lithium ions, and a non-aqueous electrolyte.
The lithium ion battery is small in size, is light in weight, and has high energy compared to secondary batteries such as a lead battery, a nickel-cadmium battery, and a nickel-hydrogen battery in the related art, and thus the lithium ion battery has been used as a power supply of a portable electronic apparatus such as a cellular phone, and a note-book type personal computer. In addition, recently, an examination has also been made for a high-output power supply of an electric vehicle, a hybrid vehicle, and an electric tool. High-speed charge and discharge characteristics have been demanded for the electrode active material of the battery that is used as the high-output power supply.
Therefore, in terms of high functionability, high capacity, low cost, rare metal free, and the like of the above-described secondary battery, as a positive electrode active material, various kinds of materials have been examined. Among these, an olivine-type phosphate-based electrode active material represented by LiFePO4 has attracted attention as an electrode active material from the viewpoints of safety, abundant resources, and low cost.
Among the phosphate-based electrode active materials, lithium manganese phosphate (LiMnPO4), in which an alkali metal is Li and a transition metal is Mn, having a problem due to lithium manganese phosphate having a theoretical capacity of approximately 170 mAh/g that is substantially the same as that of LiFePO4, but even under low-rate discharge conditions, material utilization being very poor compared to LiFePO4 has been pointed out in various documents (refer to Non-Patent Document 1 and the like).
As one problem of the poor material utilization, a problem of slowness in Li diffusion inside an active material, which is derived from a structure of a phosphate-based electrode active material, such as LiMnPO4, may be exemplified.
In the phosphate-based electrode active material, it is known that the Li diffusion inside the active material occurs only in a b-axis direction of a crystal lattice while being accompanied with phase conversion of two phases of LiMnPO4 and MnPO4 (refer to Non-Patent Document 2), and it is described that the phosphate-based electrode active material has a disadvantage for high-speed charge and discharge.
As an effective method to solve the problem, a method of shortening a crystal lattice length of LiMnPO4 particles in the b-axis direction for the purpose of shortening a Li diffusion distance in the particles, a method of enlarging crystal lattice lengths along an a-axis and a c-axis for the purpose of securing a wide Li diffusion space, and a method of making the LiMnPO4 particles fine for the purpose of increasing a reaction area between Li and LiMnPO4 particles, and the like may be exemplified.
As a method of making the LiMnPO4 particles fine, a method of making the LiMnPO4 particles fine by mechanical pulverization is general (refer to Patent Document 1 and the like).
In addition, as another method, a method of making particles fine using polyhydric alcohols such as glycols and polyols which have a high boiling point is suggested (refer to Patent Document 2 and the like).
This method is a method of allowing LiMnPO4 particles to precipitate while heating a sufficient amount of precursor in the polyhydric alcohols such as glycols and polyols which have a high boiling point.